The electrical malfunction that filled a Metro tunnel with smoke Jan. 12, killing one passenger and sickening scores of others, involved a type of power cable that the transit agency had planned to remove from the subway over the next decade, replacing it with cable that is designed to emit fewer noxious gases if it starts to burn.

The work of installing safer, “low-smoke” power cables throughout the rail network began a few years ago and had been projected to continue until the early 2020s.

However, in the three weeks since the older kind of cable was involved in the fatal tunnel calamity near the L’Enfant Plaza station, the replacement project has taken on new urgency for Metro, with engineers scrambling to devise a plan to accelerate the process, according to top managers at the agency.

The crisis occurred just south of L’Enfant Plaza, where a six-car Yellow Line train encountered heavy smoke in a tunnel, authorities said. Hazardous vapor permeated the immobile train as choking passengers waited more than 30 minutes for help to arrive. One rider, Carol I. Glover, 61, of Alexandria, died of smoke inhalation, an autopsy found.

The National Transportation Safety Board, conducting what it said will be a months-long investigation, has cited a “severe” electrical malfunction and heavy smoke in an area of the tunnel where some of the older cables were located.

As the cables’ insulation and other material burned in the tunnel, the trapped, gasping passengers inhaled an array of gases. What those gases were isn’t clear. But Metro said the newer, “low smoke/low halogen” cable is designed to reduce emissions of several irritating, even poisonous fumes during meltdowns such as the one near L’Enfant Plaza.

“This type of cabling has insulation and jacket material free of halogenated materials like fluorine, chlorine, bromine, iodine and astatine compounds, which are reported to be capable of being transformed into toxic and corrosive matter during combustion,” Metro spokesman Dan Stessel said in an e-mail.

With its different insulation, the newer cable “produces low levels of halogen gases, which have a reduced effect if inhaled,” Stessel said. “Also a low level of white smoke is generated with limited impairment of visibility.”

The older cable, made by General Cable BICC, based in Kentucky, is used for a variety of purposes in the subway system, including as “jumper cables.”

At many places in the rail system, wide gaps in the third rails are necessary. Jumper cables bridge the gaps like extension cords, ensuring that 750 volts of current flow through the third rails, providing power to trains.

If a malfunction causes the older type of cable to burn — as happened with jumper cables near L’Enfant Plaza — the cables’ smoldering insulation can emit high levels of carbon monoxide, halogen and other dangerous gases, according to combustion experts.

Metro had planned to gradually remove General Cable jumper cables as part of a much larger, long-term project in which 126 miles of steel third rails are to be replaced with third rails made of a better material. The third-rail replacement program, begun in 2011, is targeted to be completed in 2023, the transit agency said.

The National Transportation Safety Board said the smoke in the Metro tunnel near L'Enfant Plaza was caused by electrical arcing, which occurs when electricity escapes its prescribed path. Here's how that happens. (Jason Aldag/The Washington Post)

Replacing the older jumper cables “is part of the track-rehabilitation program; it’s one of the sub-projects within that project,” Rodrigo Bitar, Metro’s assistant general manager for infrastructure and engineering, said in an interview.

Before the Jan. 12 tunnel incident left more than 200 passengers caught in clouds of sickening fumes, Bitar said, “we had no reason to believe that we needed an exclusive program” for replacing older jumper cables with the newer kind, manufactured by Draka Cableteq USA, of Massachusetts.

Now, Bitar said, “what we want to do is put together a program — or see about the feasibility of putting together a program — to see how soon we can replace the jumper cables” ahead of the protracted third-rail replacement effort.

Lisa Lawson, a General Cable spokeswoman, said the company “has received no notice about our cable being involved in the fire, so therefore we have no comment.”

Sitting in his office, Bitar held two lengths of cable, each a yard long.

“The difference is the material in the insulation,” he said. “This one, the old one, when it burns, it’s like burning a piece of paper. It flames; it makes a puff. It’s combustible; it generates smoke. This one, the new one, is more like a cigar. You see very little fire. It’s consuming it inside. There’s not a big ignition that generates a lot of smoke.”

Older cables, including General Cable power lines, course throughout the subway system, serving not just as jumper cables. Among other functions, they are used as “feeder cables,” delivering electricity to the third rails from outside, the transit agency said.

In a separate project, begun in 2010, Metro has been replacing older, non-jumper power lines. The work is about half finished, with roughly 65 miles of the older cable having been removed, Stessel said. Replacing the jumper cables was made part of the longer-term third-rail project as a labor efficiency, he said.

The reason for the overall cable-replacement effort wasn’t primarily the gas-emission issue, Bitar said. “It’s part of regular maintenance,” he said, noting that power cables, like any infrastructure, have finite life spans.

Transit experts outside of Metro said they aren’t surprised that the agency had been taking a gradual approach to replacing the jumper cables before the tunnel crisis. Nor are they surprised that Metro is trying to speed up the process.

“Usually, transit systems don’t go out of their way to remove an entire component of their system just because something better has come on the market,” said Allan M. Zarembski, director of the University of Delaware’s railroad engineering and safety program. “But they do when there’s a problem like this,” he said, referring to the Jan. 12 incident.

What caused the electrical malfunction and why the tunnel filled with smoke remains under investigation. Beyond issuing a brief preliminary report and offering a few general updates, the NTSB, like Metro, has declined to comment on the inquiry. But some new details have emerged that suggest possible scenarios.

When southbound Yellow Line train No. 302 encountered smoke and stopped, moments after pulling out of the L’Enfant Plaza station, its front car was about 850 feet into the tunnel. Ground zero of the crisis, where the power-cable meltdown happened, was about 1,100 feet ahead of the lead car, on the left side of the tunnel, the side with the third rail.

There is a 20-foot gap in the third rail at that spot, to allow for safe access to an emergency exit on the left side. The gap was bridged by a bundle of four General Cable jumper cables, each 1 1/ inches in diameter. Inside each cable’s quarter-inch of insulation, about 130 copper strands carried electricity across the gap.

Most of the bundle was out of sight, encased in a square conduit running along the track bed. But some of the bundle was uncovered on each end, where the cables connected with the third rail.

The NTSB report cited “severe electrical arcing damage” to the jumper cables. Arcing suggests that electricity was escaping from one or more of the cables. This would occur if the insulation was badly deteriorated or had been damaged and the copper strands, juiced with high-voltage current, were exposed.

Without discussing specifics of the investigation, Bitar said that because of the rumble of passing trains, cables in bundles continually vibrate and rub against one another, causing insulation to wear out. Since the Jan. 12 incident, he said, Metro has been exploring ways to better secure each cable within a bundle to prevent rubbing.

“We don’t know yet” why the arcing occurred, Bitar said. “We’re waiting to find out all kinds of results from the NTSB investigation. We just want to be proactive.”

Environmental factors — including moisture, oil, grime and other corrosive substances in a subway tunnel — also can damage cable insulation.

Even with the copper strands exposed, arcing won’t occur — meaning power won’t flow out of a cable — unless the strands come in contact with a material or substance that conducts electricity, such as metal or water. The tunnel walls where the Jan. 12 malfunction occurred are lined with metal. And there were puddles of water on the track bed.

If an exposed electrical wire touches a conductive surface and current starts to flow, it can generate tremendous heat, especially over a prolonged period, and possibly ignite a conflagration. Or if there isn’t much combustible material in the area to fuel a big fire, the electrical heat can cause melting or a smoldering fire, still with plenty of smoke.

No one has publicly reported seeing flames in the tunnel, and the NTSB’s preliminary report did not mention a fire. That suggests that the smoke was not caused by a large blaze.

In terms of gaseous emissions, a prolonged smoldering fire can be more dangerous than major flames, said Stanislav I. Stoliarov, an assistant professor in the University of Maryland’s department of fire protection engineering.

“The reason being, in a regular fire, the chemicals that are produced typically have a limited toxicity,” he said. “You have a large amount of carbon dioxide in a regular fire. When you have a smoldering fire, you produce an enormous amount of carbon monoxide, which is orders of magnitude more toxic than carbon dioxide.”

The L’Enfant Plaza tunnel incident follows an uptick in smoke and fire events for Metrorail in 2014 after a decline in recent years. Last year, the subway system had 104 fire and smoke incidents, compared with 86 in 2013, Metro records show. 

Within a day or so of the Jan. 12 incident, after NTSB investigators studied the site and collected samples, Metro crews were allowed to make repairs so that passenger service could resume on that stretch of the Yellow Line.

When workers installed new jumper cables, Stessel said, they used the “low-smoke/low-halogen” kind made by Draka Cableteq USA.

Lori Aratani contributed to this report.